{
  "eventId": "GRB 250329B",
  "submittedHow": "email",
  "subject": "GRB 250329B: Swift-XRT afterglow detection",
  "body": "A.P. Beardmore (U. Leicester), J.P. Osborne (U. Leicester), M. Ferro\n(INAF-OAB), R. Brivio (INAF-OAB), A. D'Ai (INAF-IASFPA), J.A. Kennea\n(PSU), D.N. Burrows (PSU), M. A. Williams (PSU) and P.A. Evans (U.\nLeicester) reports on behalf of the Swift-XRT team:\n\nWe have analysed 1.1 ks of XRT data for the SVOM/ECLAIRs-detected burst\nGRB 250329B, from 7.4 ks to 8.5 ks after the  SVOM/ECLAIRs trigger. The\ndata are entirely in Photon Counting (PC) mode. An uncatalogued X-ray\nsource is found which is substantially brighter than an upper limit\nderived from the Rosat All-Sky Survey and is thus likely the afterglow.\n\nUsing 1140 s of PC mode data and 1 UVOT image, we find an enhanced XRT\nposition (using the XRT-UVOT alignment and matching UVOT field sources\nto the USNO-B1 catalogue): RA, Dec = 207.66921, +52.82343 which is\nequivalent to:\n\nRA (J2000): 13h 50m 40.61s\nDec(J2000): +52d 49' 24.3\"\n\nwith an uncertainty of 3.3 arcsec (radius, 90% confidence). This\nposition is 3.7 arcmin from the SVOM/ECLAIRs position. \n\nWe cannot at this time determine whether the source is fading.\n\nA spectrum formed from the PC mode data can be fitted with an absorbed\npower-law with a photon spectral index of 3.2 (+0.9, -0.8). The\nbest-fitting absorption column is  7.1 (+4.0, -3.1) x 10^21 cm^-2, in\nexcess of the Galactic value of 1.3 x 10^20 cm^-2 (Willingale et al.\n2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion\nfactor deduced from this spectrum  is 2.6 x 10^-11 (1.4 x 10^-10) erg\ncm^-2 count^-1. \n\nA summary of the PC-mode spectrum is thus:\nTotal column:\t     7.1 (+4.0, -3.1) x 10^21 cm^-2\nGalactic foreground: 1.3 x 10^20 cm^-2\nExcess significance: 3.8 sigma\nPhoton index:\t     3.2 (+0.9, -0.8)\n\nThe results of the XRT-team automatic analysis are available at\nhttp://www.swift.ac.uk/xrt_products/00019667.\n\nThis circular is an official product of the Swift-XRT team.\n\n",
  "createdOn": 1743323205810,
  "bibcode": "2025GCN.39939....1B",
  "submitter": "Phil Evans at U of Leicester <pae9@star.le.ac.uk>",
  "circularId": 39939
}